The present invention relates to a planar lighting device used for indoor and outdoor illumination and comprising light sources and a light guide plate for admitting light emitted by the light sources and emitting the light through the light exit plane thereof or to a planar lighting device used as a backlight for illuminating the liquid crystal display panel of a liquid crystal display device or as a backlight used for advertising panels, advertising towers, advertising signs, and the like.
Liquid crystal display devices use a backlight unit for illuminating the liquid crystal display panel from the rear side thereof. A backlight unit is configured using optical members comprising a light guide plate for diffusing light emitted by an illuminating light source to irradiate the liquid crystal display panel, a prism sheet, and a diffusion sheet.
Currently, large liquid crystal display televisions predominantly use a so-called direct illumination type backlight unit having no light guide plate but comprising optical members such as a diffusion plate disposed immediately above the illuminating light source. This type of backlight unit comprises cold cathode tubes serving as a light source provided on the rear side of the liquid crystal display panel whereas the inside of the backlight unit provides white reflection surfaces to secure uniform light amount distribution and a necessary brightness.
To achieve a uniform light amount distribution with the direct illumination type backlight unit, however, the backlight unit needs to have a given thickness, say about 30 mm, in a direction perpendicular to the liquid crystal display panel. While demands of still thinner backlight units are expected to grow in the future, achieving a further reduced thickness of say 10 mm or less with a backlight unit is deemed difficult in view of uneven light amount distribution expected to accompany the direct illumination type.
Among backlight units that allow reduction of thickness thereof is a backlight unit using a light guide plate whereby light emitted by illumination light sources and admitted into the light guide plate is guided in given directions and emitted through a light exit plane, which is a different plane from the plane through which light is admitted.
There has been proposed a backlight of a type described above using a light guide plate formed by mixing scattering particles for diffusing light into a transparent resin, for which reference may be had, for example, to JP 07-36037 A, JP 08-248233 A, JP 08-271739 A, and JP 11-153963 A.
JP 07-36037 A, for example, discloses a light-diffusion light-guide light source device comprising a light-diffusion light guide member having at least one light entrance plane region and at least one light exit plane region and light source means for admitting light through the light entrance plane region, the light-diffusion light guide member having a region that has a tendency to decrease in thickness with the increasing distance from the light entrance plane.
JP 08-248233 A discloses a planar light source device comprising a light-diffusion light guide member, a prism sheet provided on the side of the light-diffusion light guide member closer to a light exit plane, and a reflector provided on the rear side of the light-diffusion light guide member. JP 08-271739 A discloses a liquid crystal display comprising a light emission direction correcting element formed of sheet optical materials provided with a light entrance plane having a repeated undulate pattern of prism arrays and a light exit plane given a light diffusing property. JP 11-153963 A discloses a light source device comprising therein a light-diffusion light guide member having a light scattering property and light supply means for supplying light through an end plane of the light-diffusion light guide member.
In the planar lighting devices provided with a light-diffusion light guide plate containing light scatterers mixed therein as disclosed in the above prior art literature, light emitted by the light source and admitted through the light entrance plane into the light-diffusion light guide member receives a single or a multiple scattering effect at a given rate as the light propagates through the inside of the light-diffusion light guide member. Moreover, a significant proportion of light that reaches both end planes of the diffusion light guide member or a surface of the reflector receives reflection effect and is returned back into the diffusion light guide member.
The above composite process produces light beam that is emitted through the light exit plane highly efficiently with a directivity to travel obliquely forward as viewed from the light source. Briefly, light radiated by the light source is emitted through the light exit plane of the light-diffusion light guide member.
Thus, the prior art literature mentioned above purportedly states that a light guide plate containing scattering particles mixed therein is capable of emitting uniform light with a high light emission efficiency.
As regards the light guide plate used in the planar lighting device, there have been disclosed a light guide plate in the form of a flat plate and a light guide plate composed of a portion shaped to have a region with a tendency to grow thinner with the increasing distance from the light entrance plane attached to the other portion, in addition to the light guide plate mentioned above that is shaped to have a region with a tendency to grow thinner with the increasing distance from the light entrance plane.